I would go with the cheapest, as long as you know your car is in good condition with no problems (which most people on this site do). At the end of the day if it passes, that's all you need. The only time you need to worry is if you think there may be issues with the car, that is when certain dodgy companies may try and make a "quick buck" on you. I did my last MOT at Kwik Fit and was happy with the service, in case you are looking for any recommendations. Once again, I know many people may have had bad experiences with Kwik Fit, but I haven't, and was pretty sure that my car would pass MOT - which it did.